Polytetrafluoroethylene waveguide window construction



June 25, 1963 "r. J. KlLDUFF ETAL 3,095,550

POLYTE'I'RAFLUOROETHYLENE WAVEGUIDE WINDOW CONSTRUCTION Filed April 9, 1959 F/GZ INVENTORS TIMOTHY J K/LDUFF ASAF A. BE/VOERLY BY J}. M404 y; 1. Z 7n"6ee 9 m United States Patent POLYTETRAFLUOROETHYLENE WAVEGUIDE WINDOW CONSTRUCTION Timothy J. Kildntf, Washington, D.C., and Asaf A. Benderly, Bethesda, Md., assignors to the United States of America as represented by the Secretary of the Army p Filed Apr. 9, 1959, Ser; No. 805,350

1 Claim. (Cl. 333-98) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used. by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates to a polytetrafluoroethylene waveguide window and to an epoxy adhesive'which is used to bond the polytetnafluoroethylene window to a metal waveguide section.

In a variety of applications it is necessary to close or seal a microwave system to retain air pressure within it and to prevent the entry of water, water vapor, dust or other extraneous matter. A sealed or pressure-resistant window is generally mounted across a section of waveguide and must be essentially transparent to microwave energy over desired frequency and temperature ranges.

Commercial waveguide pressure-resistant windows are currently constructed of electrical grade glass fused to a metal frame, or of mica sealed to a metal frame with a low-melting glass or solder. Apart from their substantial cost, these windows present the diliicut problem of soldering or attachment to aluminum or brass waveguides, and also oller some risk of corrosion due to the proximity of dissimilar metals.

One of the objects of this invention, therefore, is to provide an inexpensive waveguide window which is essentially transparent to microwave energy, which is capable of operating at high temperatures, and which is able to withstand severe shock and vibration.

Another object is to provide a waveguide window which can be easily bonded across a waveguide section and which will not corrode when so bonded.

Another object of this invention is to provide an improved non corrosive and high tensile strength adhesive for bonding a film of polytetrafluoroethylene to the flange of a metallic waveguide section.

According to this invention, a polytetnafluoroethylene film is treated in a sodium-naphthalene bath to render it cementable and is then bonded to aluminum or brass waveguide flanges with a high-temperature-resistant adhesive containing epoxy resin and solvent. The film so bonded provides a waveguide window. Windows so assembled have successfully withstood pressures of about 20 p.s.i.g. at a temperature of 125 C. (257 F.) for 15 minutes and haveprovided a voltage standing wave ratio (VSWR) no greater than 1.08 when measured within a particular test-frequency band.

The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawing, in which:

FIGURE 1 shows the waveguide window of this invention prior to assembly.

FIGURE 2 shows the assembled waveguide window.

As shown in FIGURE 1, waveguide section is provided with the conventional flange 11 and a rectangular recess 12 formed in flange 11. The waveguide section 10 has the usual rectangular bore 10a extending longitudinally therethrough which is used to guide the microwave signals. The rectangular polytetrafluoroethylene window 13 is cemented by means of an epoxy adhesive 14 in recess 12. Rectangular insert 15 is insertable in recess 12 and is cemented to the edges of window 13 by epoxy- 3,095,550 Patented June 25, 1963 solvent adhesive 14. Flange 11 and insert 15 are pref e r-ably composed of brass or aluminum.

Polytetrafluoroethylene window 13 can be between 0.005-0.010 inch in thickness and should be devoid of pinholes. Windows having a thickness of 0.005 inch are preferred. Before window 13 is ready for bonding in recess 12 it is immersed for 15 minutes in a bath containing a one molar complex of sodium-naphthalene in tetrahydrofuran. The window is removed from the bath, rinsed first with acetone and then with water and gently wiped dry with a soft tissue. It is then ready to receive the epoxy adhesive 14 compounded from the following ingredients:

fiactory is Epon 815. Epon 815 is a reaction product of epichlorohydrinbisphenol-A variety, with ten percent of butyl glycidyl ether diluent. Epon 815 has a viscosity of between 6-9 poises and an epoxy equivalent of approximately 175.

Adhesive 14 is of sufficiently low viscosity to allow extremely thin layers, less than 0.0005 inch in thickness, to be painted on the window 13. Because of the waterlike consistency of the adhesive layer it has no significant effect on the electrical behavior of the window. Therefore, when the entire window is painted with adhesive it is not necessary to mask the transmission area of the window. In addition, this thin layer provides a strong high-temperature bond for bonding the window 13 to the recess 12 as well as for bonding the brass or aluminum insert 15 to the brass or aluminum flange 11.

In assembling a typical waveguide window in accordance with this invention, a thin layer of adhesive 14 of approximately 0.0005 inch in thickness is painted over the entire side 13a of window 13. Window 13 consists of a rectangular film of polytetrafluoroethylene 0.005 inch in thickness. Window 13 is dropped in place in the brass or aluminum recess 12, adhesive side facing rectangular bore 10a. Additional adhesive 14 is applied to the edges of the aluminum or brass insert 15 and to the face of the insert 15 which is to be pressed against side 13b of window 13. Thus insert 15 will bond to side 13b of window 13 and to recess 12.

If desired, the window and insert which are coated with adhesive 14 can be dried at room temperature for 15 rninutes and then at 65 degrees centigrade in a forceddraft oven for one hour. The parts will then be dry to touch and can be stored for many days prior to the assembly and final cure.

After the window 13 is placed in recess 12 and insert 15 placed against the face 13b of the window, 15 to 20 p.s.i. pressure is applied to the insert 15 by a spring or other suitable means which can be devised by those skilled in the art. Curing of the adhesive 14 under pressure of 15 to 20 p.s.i. is effected at a temperature of 180 degrees cen-tigrade in a forced-draft oven for 3 hours.

A cured assembly such as the one described above will confine an air pressure of 20 p.s.i.g. in rectangular bore 10a when the section 10 is heated to a temperature of degrees centigrade for 15 minutes. The insertion of the 0.005 inch-thick polytetnafluoroethylene window across the rectangular bore 10a does not increase the voltage standing wave ratio (VSWR) beyond 1.08 when measured within a band consisting of 2.00 megacycles on either side of a particular microwave frequency. Tests indicate that the VSWR varies insignificantly between frequencies 3 of 8,000 and 10,000 megacycles. The value of 1.08 can be compared with values of VSWR in excess of 1.10 which are typical of the substantially more expensive commercial glass or mica windows.

In light of the foregoing it will be evident to those in the art that waveguide windows composed of po-lytetrafluoroethylene and constructed as described have advantages not attainable by known windows. In addition, an epoxysolvent adhesive is disclosed which has an extremely low viscosity and which is resist-ant to high temperatures resulting from heat developed by the flight of the missile in which the waveguides are incorporated.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claim.

We claim as our invention:

In a waveguide assembly comprising a metallic section, a bore through said section substantially coaxial to the longitudinal axis of said section, a flange formed on one end of said section, a recess formed in said flange, the improvement comprising: a waveguide window in said recess, said window consisting of a film of polytetrafluoroethylene, said film having a bondable surface portion on all planar surfaces thereof, and an adhesive covering substantially the entire area of that planar surfiace of said film which is in contact with said waveguide and bonding said film in said recess, said adhesive comprising epichlorhydrin bis-(4-hydroxyphenyl)-2,2-propane and an acid anhydride.

References Cited in the file of this patent UNITED STATES PATENTS 2,637,776 Edson May 5, 1953 2,786,794 Gams et a1 Mar. 26, 1957 2,788,306 Cox Apr. 9, 1957 2,839,495 Carey June 17, 1958 2,882,502 Freundlich Apr. 14, 1959 2,883,308 Yarnada Apr. 21, 1959 2,932,806 Burr Apr. 12, 1960 2,947,325 McFarland Aug. 2, 1960 3,001,160 Trousdale Sept. 19, 1961 FOREIGN PATENTS 209,032 Australia June 19, 1957 OTHER REFERENCES Lee et al.: Epoxy Resins, page 142, McGraw-Hill Book Co., Inc., New York. Copyright 1957.

Lee et al.: Epoxy Resins, page 198, McGraw-Hill Book Co., Inc., New York. Copyright 1957, TP986.E6.

Publication, Ep0x-iesNo Wonder, published in Modern Plastics. October 1952. Pages 89-94. 

